The ability to convert optical energy efficiently from one spatial mode to another has potential applications in optical fiber-based devices such as wavelength filters, sensors, and dispersion compensators. A variety of grating-based mode converters have been demonstrated using periodic stress, microbending, and photo-induced index changes. (See, for example, "Fiber-optic modal coupler using periodic microbending", by J. N. Blake et al., Optics Letters, March 1986, Vol. 11, pp. 177-179.)
The first two of these techniques have the potential for high conversion efficiency and low insertion loss, but suffer from poor long-term reliability. Mode converters based on photo-induced index gratings promise both high reliability and performance and, thus, have been the focus of research in recent years. (See "Intermodal coupler using permanently photoinduced grating in two-mode optical fiber", by H. G. Park et al., Electronics Letters, Jun. 8, 1989, Vol. 25, pp. 797-799.)